Violation of multipartite Bell inequalities with classical subsystems via operationally local transformations

TL;DR

This paper generalizes the violation of Bell inequalities using local transformations to multipartite systems, showing that such violations can occur even with classical systems if the ancillary system violates Bell inequalities.

Contribution

It extends the framework of operationally local transformations to multipartite Bell inequalities and establishes conditions under which violations imply nonlocality.

Findings

Violation depends on the ancillary system violating Bell inequalities.

System under test can be classical yet violate Bell inequalities.

Nonlocality may reside in environment or measurement settings.

Abstract

Recently, it was demonstrated by Son et al., Phys. Rev. Lett. \textbf{102}, 110404 (2009), that a separable bipartite continuous variable quantum system can violate the Clauser-Horne-Shimony-Holt (CHSH) inequality via operationally local transformations. Operationally local transformations are parameterized only by local variables, however in order to allow violation of the CHSH inequality a maximally entangled ancilla was necessary. The use of the entangled ancilla in this scheme caused the state under test to become dependent on the measurement choice one uses to calculate the CHSH inequality thus violating one of the assumptions used in deriving a Bell inequality, namely the `free will' or `statistical independence' assumption. The novelty in this scheme however is that the measurement settings can be external free parameters. In this paper we generalize these operationally local transformations for multipartite Bell inequalities (with dichotomic observables) and provide necessary and sufficient conditions for violation within this scheme. Namely, a violation of a multipartite Bell inequality in this setting is contingent on whether an ancillary system admits any realistic, local hidden variable model i.e. whether the ancilla violates the given Bell inequality. These results indicate that violation of a Bell inequality performed on a system does not necessarily imply that the system is nonlocal. In fact the system under test may be completely classical. However, nonlocality must have resided somewhere, this may have been in the environment, the physical variables used to manipulate the system or the detectors themselves provided the measurement settings are external free variables.